German patent DE-PS 28 02 676 describes a closing device for clutches running in an oil bath. The dosing device is provided with a slider which cooperates with a piston in such a manner that upon engagement of the piston on respective friction disks, the slider opens an internal channel within the shaft so as to supply additional oil pressure to the friction disks.
This arrangement has, however, several disadvantages. Since an internal channel is provided in the shaft, the strength of the latter is reduced and can be compensated only by overdimensioning the shaft, i.e. by providing the shaft with a larger diameter. This leads however to the need for larger bearings which is contrary to the desire to provide a relatively small structural unit.
Moreover, the piston stroke is considerable in order to sufficiently open the slider, a fact which again leads to a larger structural unit. Apart from this, a larger piston stroke demands a larger amount of pressure fluid which may not be available when referring to hydraulic actuation of the foot pedal without a displacement-ratio transmission in the pedal path. Finally, the earlier device does not prevent oil moistening of the friction disks when no actuation occurs so that energy-consuming drag torque will be present continuously.